Reverse osmosis

Short definition

Reverse osmosis (RO) is the drinking-water filtration method that pushes water through a semi-permeable membrane to remove 95–99% of dissolved solids, including lead, copper, arsenic, nitrates, fluoride, and pathogens. Standard residential RO is a 4–5 stage under-sink system at the kitchen — point-of-use, not whole-house. Best for lead, heavy metals, and TDS reduction.

What it is

In normal osmosis, water moves from low-solute concentration to high-solute concentration through a membrane until concentrations equalize. Reverse osmosis applies pressure to push water the opposite direction, leaving dissolved solids behind. The result is highly purified water on one side of the membrane and a concentrated “reject” stream on the other.

Components of a typical residential RO system:

1. Sediment pre-filter (5-micron) — protects everything downstream.
2. Carbon pre-filter — chlorine and chloramine pre-treatment to protect the membrane (chloramine destroys RO membranes; catalytic carbon required for chloraminated water).
3. RO membrane — the core component; typically 50–100 GPD residential rating.
4. Storage tank — pressurized 3–4 gallon tank for instant access at the faucet.
5. Polishing carbon — post-membrane taste improvement.
6. Air-gap RO faucet — required by UPC and WA cross-connection rules; prevents drain backflow into the RO.
7. Drain saddle — connects the reject line to the drain.

The cross-connection rule (WAC 246-290-490) requires an air gap between the RO drain saddle and the drain itself — the air-gap faucet handles this for under-sink installs.

What RO removes:
– 95–99% of total dissolved solids (TDS).
– Lead, copper, mercury, cadmium, chromium, arsenic.
– Nitrates, sulfates, fluoride.
– Most pathogens (bacteria, viruses, protozoa).
– Pharmaceutical residues (many).

What RO doesn’t remove well:
– Some VOCs (the carbon pre/post-filter handles these).
– Dissolved gases (radon, hydrogen sulfide).

Typical wastewater rate: 2–4 gallons “reject” per gallon produced. Newer systems with permeate pumps reduce this; tankless RO systems also have higher efficiency.

Why it matters to a homeowner

For drinking water, RO is the gold-standard residential treatment for the broadest range of contaminants. It’s the right tool for:

• Lead removal in pre-1986 homes — RO is highly effective for lead.
• Heavy metals in well water — Olympic Peninsula arsenic, San Juan saltwater intrusion, Skagit nitrate.
• Sodium removal post-softener — softeners add a small amount of sodium during ion exchange; RO at the drinking tap removes it.
• High TDS water in some private wells.

Cost: $200–$600 for the equipment, $200–$500 for typical install. Total $400–$1,100 for a quality under-sink system.

Practical limits:

• Wastewater. Standard RO produces 2–4 gallons of reject water per gallon of clean water. In drought-conscious WA areas, this matters. Newer permeate-pump and tankless systems reduce the reject ratio.
• Slow flow. RO produces water slowly (typically <1 GPM at the dedicated faucet). The 3–4 gallon storage tank handles bursts.
• Cartridge replacement. Pre-filters every 6–12 months, polishing filter annually, membrane every 2–5 years.

When a contractor recommends “whole-house RO” — slow down. Whole-house RO is expensive, slow, and typically only justified for very high TDS or arsenic levels in wells. POU RO at the kitchen handles drinking water at a fraction of the cost.

Common failure modes

• Membrane fouling by chlorine or chloramine — use catalytic carbon pre-filter for chloraminated water (Seattle Public Utilities specifically).
• Pre-filter overdue — membrane premature failure; replace pre-filters every 6–12 months.
• Storage tank bladder loss — no pressure; drip flow only at the faucet.
• Air-gap faucet drain blocked — RO won’t produce.
• Reject line freeze — if installed in unconditioned space.
• Self-piercing saddle valve at the supply — leaks and fails over time. Better to use a tee with a proper supply stop.

Common variants

• Standard RO (50 GPD, residential) vs. tankless RO (no storage, on-demand, more efficient).
• 4-stage vs. 5-stage vs. 6-stage (additional remineralization for taste).
• POU RO under kitchen sink vs. POE whole-house RO (rare; expensive; usually only for very high TDS).
• RO vs. distillation (similar performance, different mechanism).
• RO vs. carbon block (RO removes dissolved; carbon doesn’t).

Washington note

A few WA-specific RO considerations:

Seattle chloraminated water — SPU uses chloramine. Standard activated carbon pre-filters won’t fully protect the RO membrane from chloramine; specify catalytic activated carbon for the pre-filter stage.

Olympic Peninsula well water — combined arsenic, iron, and pH issues often require multi-stage POE filtration ahead of the POU RO. RO alone can handle arsenic, but the membrane life suffers if upstream water isn’t pre-treated.

Pre-1986 Seattle home with first-draw lead positive — POU RO at the kitchen for drinking and cooking. Confirm post-install with a follow-up lab lead test.

Pre-1990 home with soft acidic water and concern about copper — RO at the sink as drinking-water polish. Address the upstream copper-pinhole risk separately (pH neutralizer at POE if well-fed).

Spokane home with softener — RO at the kitchen removes the sodium added during softening, useful for sodium-restricted diets.

Tiny home or RV — small-volume tankless RO is popular for compact off-grid drinking water.

For pre-purchase scopes, an existing under-sink RO is a positive — but verify the cartridges are current and the system isn’t bypassed. A neglected RO is worse than no RO.